@philipfaster how did you get this working? It looks great on your website.
I uploaded the js Reiner refered to:
https://github.com/foxcreation/FOXY_solutions/blob/master/wp-content/themes/foxy-illdy/layout/js/tableOfContentActiveScroll.js
I then added the functions.php script to my child theme:
https://github.com/foxcreation/FOXY_solutions/blob/e897070d8d3e0050238158dcce4e553d5bc644c1/wp-content/themes/foxy-illdy/functions.php
Then I added your CSS style to my child theme in your post above.
But it doesnt highlight on active state. What am I doing wrong?